In a conventional multi-coin validator coins pass along a path past a number of sensor coils which are energized to produce an inductive coupling with the coin. The degree of interaction between the coin and the coil is a function of the relative size of the coin and coil, the material from which the coin is made and also its surface characteristics. Thus, by monitoring the change in impedance presented by each coil, data indicative of the coin under test can be provided. The data can be compared with information stored in the memory to determine coin denomination and authenticity.
In U.S. Pat. No. 4,754,862, assigned to the assignee of the present application, there is described coin discrimination apparatus utilising a plurality of inductive sensor coils which are each included in a respective resonant circuit. The resonant circuits are driven by a variable frequency oscillator through a multiplexer. As the coin passes a particular coil, the natural resonant frequency of the resonant circuit is altered due to the inductive coupling between the coin and the coil. The circuit is maintained at its natural resonant frequency by means of a phase locked loop which alters the frequency of the oscillator so as to track the natural resonant frequency of the resonant circuit during passage of a coin past coil. As a result, the amplitude of the oscillatory signal developed across the resonant circuit varies substantially on a transitory basis. The amplitude deviation produced by passage of the coin past the coil is a function of coin denomination. It has been found that by using three coils of different sizes and configurations, three coin signals can be provided which uniquely characterize coins of a particular coin set e.g. the UK coin set.
The amplitude deviations produced by the three coils are digitized to produce the coin signals and are then compared with reference values stored in a programmable memory in order to discriminate between coins of different denominations, and frauds.
The disadvantage of this prior art arrangement is that the use of a voltage controlled oscillator and phase locked loop consumes substantial amounts of electrical power.